Elements And Compounds Around Us Class 8th Science Term 1 Tamilnadu Board Solution

Class 8th Science Term 1 Tamilnadu Board Solution

Evaluation
Question 1.

Water is made up of two elements, hydrogen and oxygen. Water is a liquid, whereas hydrogen and oxygen are gases. Hydrogen catches fire easily. Oxygen helps in burning. Water is used to put off the fire.

From the above information, answer the following questions.

a) What are the elements present in water?

b) In which state do these elements exist?

c) Write the property of hydrogen.

d) Write the property of oxygen.

e) Do the properties of water differ from hydrogen and oxygen?


Answer:

a) Water is made up of two elements i.e. hydrogen & oxygen.

Water is a compound of two hydrogen atoms and one oxygen atom in the ratio 2:1 by volume.


2H2 + O2→ 2H2O



b) Hydrogen & oxygen exists in gases state.


Hydrogen & oxygen exists in gases state at standard temperature and pressure. This is because the intermolecular forces of attraction between the particles are weak.


c) Hydrogen is colourless, odourless gas and it is lighter than air. It is highly flammable it readily catches fire.



d) Oxygen is colourless, odourless & tasteless gas. Oxygen is not flammable; it makes other things ignite at low temperature. But oxygen itself does not catch fire.



e) Yes, the properties of water are different from its constituent particles, i.e. hydrogen & oxygen.


Water is made up of two elements hydrogen & oxygen and is called a compound. Usually the properties of compounds are different from the properties of their constituent elements. Water exists in a liquid state & it acts as a fire extinguisher whereas hydrogen is highly inflammable gas and oxygen supports combustion.



Question 2.

Using the elements enclosed in the bottle, frame formulae for some compounds.



Answer:

1. Components= Hydrogen & Oxygen

Formulae= H2O


Explanation: Water is a compound of two hydrogen atoms and one oxygen atom in the ratio 2:1 by volume.


2H2 + O2→ 2H2O


2. Components = Sodium & chlorine


Formulae = NaCl


Explanation: One sodium atom combines with one chlorine atom to form sodium chloride.


Na + Cl → NaCl


3. Components = Carbon, hydrogen & oxygen


Formulae = C12H22O11


Explanation: Sugar is a compound which is made up of 12 carbon atoms, 22 hydrogen atoms & 11 oxygen atoms.


4. Components = Sodium, carbon, hydrogen & oxygen


Formulae = NaHCO3


Explanation: Baking soda is sodium hydrogen carbonate


Na2CO3 + CO2 + H2O → 2NaHCO3


5. Components = Calcium, oxygen & chlorine


Formulae = CaOCl2


Explanation: Bleaching powder is formed by passing chlorine gas over slaked lime


Ca(OH)2 + Cl2 → CaOCl2 + H2O


6. Components = Calcium & oxygen


Formulae = CaO


Explanation: Calcium oxide is formed by thermal decomposition of calcium carbonate.


CaCO3→ CaO + CO2


7. Components = Calcium, oxygen & hydrogen


Formulae = Ca(OH)2


Explanation: It is obtained by combining calcium oxide with water.


CaO + H2O → Ca(OH)2


8. Components = Calcium, carbon & oxygen


Formulae = CaCO3


Explanation: Calcium carbonate is formed by passing carbon dioxide into a solution of calcium hydroxide.


Ca(OH)2 + CO2→ CaCO3 + H2O


9. Components = Magnesium & chlorine


Formulae = MgCl2


Explanation: It indicates two atoms of chlorine chemically combined with magnesium to form magnesium chloride.


Mg + Cl2→ MgCl2


10. Components = Magnesium & oxygen


Formulae = MgO


Explanation: It indicates one oxygen molecule chemically combined with magnesium to form magnesium oxide.


2Mg + O2→ MgO


11. Components = Potassium & chlorine


Formulae = KCl


Explanation: One potassium atom combines with one chlorine atom to form potassium chloride.


K + Cl → KCl


12. Components = Hydrogen & chlorine


Formulae = HCl


Explanation: One hydrogen atom combines with one chlorine atom to form hydrogen chloride.


H + Cl → HCl


13. Components = Calcium & Chlorine


Formulae = CaCl2


Explanation: One calcium atom combines with two chlorine atom to form calcium chloride.


Ca + Cl2→ CaCl2



Question 3.

Find the valency of the underlined element in the given formulae.

i) H2O ii) KCl iii) Al2O3 iv) Fe2O3 v) CH4


Answer:

i) H2O

Valency of oxygen is 2


Let the valency of oxygen is x


We know that,


Charge on hydrogen is +1


Therefore, 2⨉1 + x = 0


2+x=0


x= -2


As the picture shows,



ii) KCl


Valency of potassium is 1


Let the valency of potassium is x


We know that,


Charge on chlorine is -1


Therefore, x + (-1) = 0


x – 1 =0


x= 1


As the picture shows,



iii) Al2O3


Valency of aluminium is 3


Let the valency of aluminium is x


We know that,


Charge on oxygen is -2


Therefore, 2⨉x + 3⨉(-2) = 0


2x – 6 =0


2x= 6


x = 


x = 3


iv) Fe2O3


Valency of iron is 3


Let the valency of iron is x


We know that,


Charge on oxygen is -2


Therefore, 2⨉x + 3⨉(-2) = 0


2x – 6 =0


2x= 6


x = 


x = 3


v) CH4


Valency of carbon is 4


Let the valency of carbon is x


We know that,


Charge on hydrogen is +1


Therefore, x+4⨉1 = 0


x+4=0


x= -4



Question 4.

Write the chemical name of the given formulae.

i) MgO ii) HCl iii) NH3 iv) ZnO v) NaCl


Answer:

i) MgO

Magnesium oxide


Magnesium combines with oxygen molecule to form magnesium oxide.


2Mg + O2→ 2MgO


Magnesium + Oxygen → Magnesium oxide


ii) HCl


Hydrochloric Acid


Hydrogen combines with chlorine to form hydrochloric acid.


H + Cl → HCl


Hydrogen + Chlorine → Hydrochloric Acid


iii) NH3


Ammonia


One nitrogen molecule combines with three hydrogen molecules to form ammonia.


N2 + 3H2→ NH3


Nitrogen + Hydrogen → Ammonia


iv) ZnO


Zinc oxide


Zinc combines with oxygen to form zinc oxide.


2Zn + O2→ 2ZnO


Zinc + Oxygen → Zinc oxide


v) NaCl


Sodium chloride


Sodium combines with chlorine to form Sodium chloride.


Na + Cl → NaCl


Sodium + Chlorine → Sodium chloride



Question 5.

Write the names of the planets (Greek God) after which these elements are named.

a) Plutonium b) Neptunium c) Uranium


Answer:


The following diagram shows the solar system:



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